Add tests and implement boundary smoothing and cutout functionality

- Introduced tests for boundary smoothing in `test_boundary_smoothing.py`, verifying that smoothing reduces stair-step artifacts while preserving boundary integrity.
- Added tests for cutout boundary behavior in `test_cutout_boundary.py`, ensuring proper handling of boundary triangles and normal blending.
- Created `test_temporary_attributes.py` to check the cleanup of temporary attributes across depth modifier branches, ensuring no warnings are raised.
- Implemented point culling functionality in `test_validity.py`, confirming that invalid points are removed without affecting adjacent geometry.
- Developed boundary smoothing logic in `boundary_smoothing.py`, allowing for depth smoothing while maintaining edge integrity.
- Added cutout boundary management in `cutout_boundary.py`, including functions for removing boundary triangles and tapering thickness profiles.
- Implemented validity checks in `validity.py` to ensure proper deletion of invalid vertices and associated wire edges.

tests/tools/nodes/test_depth_strip_cleanup.py

@@ -134,7 +134,8 @@ def test_split_input_precedes_smooth_without_cleanup_switch(kind):
     split = inputs[names.index("Split Threshold")]
     assert split.parent == inputs[names.index("Smooth")].parent
     assert split.parent.name != "Options"
-    assert (split.default_value, split.min_value, split.max_value, split.subtype) == (0, 0, 1, "FACTOR")
+    assert split.default_value == pytest.approx(0.1 if kind == "cutout" else 0)
+    assert (split.min_value, split.max_value, split.subtype) == (0, 1, "FACTOR")
     assert "Remove Strip Faces" not in names
 
 
@@ -198,7 +199,7 @@ def test_zero_split_bypasses_cleanup_and_positive_split_can_empty_mesh(kind):
     np.testing.assert_array_equal(restored, original)
 
 
-def test_cutout_cleans_triangle_strips_only_when_split_is_enabled():
+def test_cutout_removes_an_all_boundary_triangle_strip():
     import bmesh
     from tests.support.depth_surface import surface, evaluated
 
@@ -208,15 +209,10 @@ def test_cutout_cleans_triangle_strips_only_when_split_is_enabled():
     bmesh.ops.triangulate(mesh, faces=list(mesh.faces))
     mesh.to_mesh(obj.data)
     mesh.free()
-    set_value("Split Threshold", 0.0)
-    original, faces = evaluated(obj)
-    assert len(faces) == 4
-    set_value("Split Threshold", 0.001)
-    assert not evaluated(obj)[1]
-    set_value("Thickness", 0.2)
-    assert not evaluated(obj)[1]
-    set_value("Thickness", 0.0)
-    set_value("Split Threshold", 0.0)
-    restored, restored_faces = evaluated(obj)
-    assert restored_faces == faces
-    np.testing.assert_array_equal(restored, original)
+    # Every vertex lies on the boundary, even before any depth separation.
+    for split in (0.0, 0.001, 0.0):
+        set_value("Split Threshold", split)
+        for thickness in (0.0, 0.2):
+            set_value("Thickness", thickness)
+            points, faces = evaluated(obj)
+            assert not faces and not len(points)